Water-swellable hydrophilic polymers

ABSTRACT

Water-swellable hydrophilic polymers which contain polymers prepared by free radical (co)polymerization of one or more hydrophilic monomers of the formula I ##STR1## wherein R 1  is hydrogen, methyl or ethyl, 
     R 2  is the group --COOR 4 , the sulphonyl group, the phosphonyl group, the phosphonyl group esterified by (C 1  -C 4 )-alkanol or a group of the formula ##STR2## R 3  is hydrogen, methyl, ethyl or the carboxyl group, R 4  is hydrogen, amino or hydroxy-(C 1  -C 4 )-alkyl and 
     R 5  is the sulphonyl group, the phosphonyl group or the carboxyl group, or grafting (co)polymerization of one or more hydrophilic monomers of the formula I onto a grafting base, using a free radical initiator which forms three or more free radical sites per molecule.

The present invention relates to water-swellable hydrophilic polymerswhich can be prepared by free radical polymerization using free radicalinitiators which form di- or poly-radicals.

Water-swellable hydrophilic polymers, in particular crosslinked polymersand copolymers based on acrylic or methacrylic acid,acrylamidopropanesulphonic acid copolymers or graft polymers on starchor polyalkylene oxides, have been known for a long time and aredescribed, for example, in U.S. Pat. Nos. 4,931,497, 5,011,892,5,041,496, 3,926,891 and the literature references cited therein.

They can absorb several times their weight of water or aqueous liquids,such as urine or blood, and are therefore employed as absorbents, inparticular in hygiene articles such as nappies for babies andincontinence pants for adults, and also tampons and the like.

Such water-swellable hydrophilic polymers are as a rule prepared by freeradical polymerization in an aqueous solution which contains themonomers and, if appropriate, grafting base and crosslinking agent. Thepolymerization can be initiated by high-energy radiation and/orchemically. Chemical initiators which are employed here are, forexample, peroxide compounds, such as peroxodisulphates, hydrogenperoxide, benzoyl peroxide, tert.-butyl hydroperoxide or tert.-butylperpivalate, azo initiators, such as 2,2'-azobis(isobutyronitrile)(AIBN) or 2,2'-azobis(2-amidinopropane) dihydrochloride, or redoxsystems, such as, for example sodium peroxodisulphate/sodiumpyrosulphite or hydrogen peroxide/hydroxylamine chloride. Benzoin,benzil and derivatives thereof or acetophenone derivatives can also beused as photo-initiators. All these initiators have the common featurethat they form mono-radicals which trigger off the polymerization.

However, the products prepared in this manner have various networkdefects which are due to unwanted secondary reactions during thepolymerization and which adversely influence the properties of theproducts. For example, oligomers are formed which are not incorporatedinto the polymeric network and therefore can be extracted from theswollen network and are thus inactive constituents. In addition, polymerchains which are bonded to the network on only one side are alsoinactive.

It has now been found that the network defects mentioned can be avoidedor largely avoided if compounds which instead of forming only one freeradical site per molecule, such as the compounds previously employedaccording to the prior art, form two or more free radical sites permolecule are employed as the free radical initiators.

The present invention thus relates to water-swellable hydrophilicpolymers which can be prepared by free radical (co)polymerization of oneor more hydrophilic monomers or graft (co)polymerization of one or morehydrophilic monomers onto a suitable grafting base, characterized inthat a free radical initiator which forms two or more free radical sitesper molecule is employed.

Suitable hydrophilic monomers are, for example, acids which are capableof polymerization, such as acrylic acid, methacrylic acid, caproic acid,vinylsulphonic acid, vinylphosphonic acid, maleic acid, including theanhydride thereof, fumaric acid, itaconic acid,2-acrylamido-2-methylpropanesulphonic acid and their amides,hydroxyalkyl esters and esters and amides containing amino groups orammonium groups. Water-soluble N-vinylamides or elsediallyldimethylammonium chloride are furthermore suitable.

Preferred hydrophilic monomers are compounds of the general formula I##STR3## wherein

R¹ is hydrogen, methyl or ethyl,

R² is the group --COOR⁴, the sulphonyl group, the phosphonyl group, thephosphonyl group esterified by (C₁ -C₄)-alkanol or a group of theformula ##STR4##

R³ is hydrogen, methyl, ethyl or the carboxyl group,

R⁴ is hydrogen, amino or hydroxy-(C₁ -C₄)-alkyl and

R⁵ is the sulphonyl group, the phosphonyl group or the carboxyl group.

Particularly preferred hydrophilic monomers are acrylic acid andmethyacrylic acid.

Suitable grafting bases can be of natural or synthetic origin. Examplesare starch, cellulose or cellulose derivatives and other polysaccharidesand oligosaccharides, polyalkylene oxides, in particular polyethyleneoxides and polypropylene oxides, and hydrophilic polyesters.

Starch and polyethylene oxides and polypropylene oxides, in particularthose described in U.S. Pat. Nos. 4,931,497, 5,011,892 and 5,041,496,are preferred. The content of these patents is also expressly aconstituent of the present disclosure.

All compounds which form two or more free radical sites per moleculewith or without the action of additional activators, such as light,radiation, heat, ultrasound, redox agents and the like, can in principlebe employed as free radical initiators. This means that these freeradical initiators can contain two, three or more groups which can formthe radicals. The free radical sites can be formed here at the sametime, but as a rule they are formed at different times, i.e. insuccession. Compounds which contain at least two hydroperoxide units,peroxide units or azo units, for example, are suitable.

Compounds having two hydroperoxide units are, in particular,diisopropylbenzene dihydroperoxide (U.S. Pat. No. 2,715,646) and2,5-dimethylhexane 2,5-dihydroperoxide. Suitable polyhydroperoxides canbe obtained, for example, by anodic oxidation of polycarboxylic acids,in particular of polyacrylic acid and polymethacrylic acid, in thepresence of oxygen (J. Pol. Sci. Volume XXXIV, pages 287 to 307 (1959)).

Peroxide units can be present, for example, as percarbonate or asperketal or perester units. Examples of such compounds are, inparticular, dioxetane compounds and tert.-butyl peresters, such as, forexample, methyl acrylate/tert.-butyl peracrylate copolymers (J. Pol.Sci. Volume XXXIV, page 301 (1959)). Polymeric peroxy-esters furthermorecan be obtained by reaction of dicarboxylic acid dichlorides withbishydroperoxides (EP-A 461 767).

Suitable compounds having several peroxide or hydroperoxide units andsyntheses thereof furthermore are described in "The Chemistry ofFunctional Groups, Peroxides", edited by S. Patai 1983, John Wiley &Sons Ltd., Chapter 13, by Ray Ceresa. The content of this publication isexpressly a constituent of the present disclosure.

It is preferable to employ free radical initiators containinghydroperoxide or peroxide units together with reducing agents. Suitablereducing agents are, for example, Fe²⁺, ascorbic acid, sulphinic acids,sulphites and formamidinesulphinic acids and salts thereof.

Suitable compounds which contain two or more azo units are, for example,reaction products of

a) azodicarboxylic acids with compounds which contain more than oneoxirane function. Di-, tri- to oligo-compounds and polymers can beobtained in this manner, depending on the oxirane compound used.

A preferred azodicarboxylic acid is, in particular,4,4'-azobis(4-cyanovaleric acid), which forms suitable free radicalinitiators, for example, with ethylene glycol diglycidyl ether or withpolyglycerol polyglycidyl ethers.

b) Hydroxyl- and amino-functional azo compounds with compounds whichcontain more than one oxirane or isocyanate group.

Suitable azo compounds are, for example,2,2'-azo-bis(N,N-dimethyleneisobutyramidine) or the correspondingdihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride,2,2'-azobis(2-methyl-N-(1,1-bis(hydroxymethyl)-2-hydroxyethyl)propionamide),2,2'-azobis(2-methyl-N-(1,1-bis(hydroxymethyl)-ethyl)propionamide) or2,2'-azobis(2-methyl-N-(2-hydroxyethyl)-propionamide), which formsuitable free radical initiators, for example with the glycidyl ethersmentioned above under a) or with hexamethylene diisocyanate, tolylenediisocyanate or phenylene diisocyanate.

c) Azobisamides with aldehydes. A suitable azobisamide is, inparticular, 2,2'-azobis(isobutyramide) dihydrate, which forms suitablefree radical initiators, for example, with formaldehyde or glyoxal.

d) Azobisnitriles with polyalcohols. In particular, reaction products of2,2'-azobisisobutyronitrile with ethylene glycol, butane-1,4-diol orhexane-1,6-diol are preferred (Makromol. Chem. 178 2533 (1977)).

Suitable initiators are, in addition, polyfunctional photoinitiators,compounds obtained by reaction of Ce⁴⁺ with polyfunctional alcohols,such as polyvinyl alcohol or cellulose (J. Po. Sci. Volume XXXI, page242 et seq. (1958)) and ozonized starch (Chemistry and Engineering News,37 27, 41 (1959)).

The free radical initiators mentioned can be used by themselves or asany desired mixtures with one another for the preparation of thehydrophilic polymers according to the invention.

They are preferably employed here in amounts of 0.001 to 20% by weight,based on the total monomers. 0.05 to 3.0% by weight is particularlypreferred.

In a particular embodiment of the present invention, free radicalinitiators in which the functions which form free radicals havedifferent reactivities or are activated by different mechanisms areused. Such initiators thus contain, for example, both azo and peroxideor hydroperoxide functions, which are activated in succession in apredetermined manner and can thus be used, for example, for thepreparation of block polymers.

It may furthermore be of advantage to use initiators in which thefunctions which form free radicals lie at different spatial distancesfrom one another in the molecule.

The molecular weight of the initiators which can be used for thepreparation of hydrophilic polymers according to the invention can ofcourse vary within wide limits. The molecular weights are, inparticular, in the range from 100 to 10,000,000.

The hydrophilic polymers according to the invention can also be preparedusing suitable crosslinking agents, i.e. compounds having at least twodouble bonds, which can be polymerized into the polymer network.

The use of crosslinking agents is particularly preferred if the freeradical initiators used form only two free radical sites per moleculeand therefore do not themselves have crosslinking properties.

On the other hand, free radical initiators which form three or more freeradical sites per molecule themselves have crosslinking properties, sothat in these cases the crosslinking agents mentioned can also bedispensed with. Nevertheless, the crosslinking agents mentioned can alsobe used in combination with initiators which form three or more freeradical sites per molecule.

Suitable crosslinking agents are, in particular, methylenebisacrylamideand -methacrylamide, esters of unsaturated mono- or polycarboxylic acidswith polyols, such as diacrylate or triacrylate, for example butanedioldiacrylate or dimethacrylate or ethylene glycol diacrylate ordimethacrylate, trimethylolpropane triacrylate and vinyl methacrylate,and allyl compounds, such as allyl (meth)acrylate, triallyl cyanurate,diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid andvinylphosphonic acid derivatives, such as are described, for example, inEP-A 343 427. The content of EP-A 343 427 is also expressly aconstituent of the present disclosure.

The crosslinking content is preferably 0 to 20% by weight, particularlypreferably 0 to 3% by weight, based on the total monomer content.

Furthermore, the hydrophilic polymers according to the invention can bepost-crosslinked in the aqueous gel phase in a manner known per se orcrosslinked on the surface in the form of ground and sieved polymerparticles. Crosslinking agents which are suitable for this are compoundswhich contain at least two groups which can form covalent bonds with thecarboxyl groups of the hydrophilic polymer. Suitable compounds are, forexample, di- or polyglycidyl compounds, such as phosphonic aciddiglycidyl ester, alkoxysilyl compounds, polyaziridines, polyamines orpolyamidoamines, it also being possible for the compounds mentioned tobe used as mixtures with one another (see, for example, EP-A 83 022,EP-A 543 303 and EP-A 530 438). Polyamidoamines which are suitablecrosslinking agents are described, in particular, in EP-A 349 935. Thecontent of the abovementioned patent applications is expressly also aconstituent of the present disclosure.

The hydrophilic polymers according to the invention can be prepared byknown polymerization processes. Polymerization in aqueous solution bythe process of so-called gel polymerization is preferred. In thisprocess, 15 to 50% strength by weight aqueous solutions of one or morehydrophilic monomers and, if appropriate, a suitable grafting base arepolymerized in the presence of a free radical initiator which can formdi- or poly-radicals, preferably without mechanical mixing and utilizingthe Trommsdorff-Norrish effect (Bios Final Rep. 363.22; Makromol. Chem.1, 169 (1947)).

The polymerization reaction can be carried out in the temperature rangebetween 0° C. and 130° C., preferably between 10° C. and 100° C., eitherunder normal pressure or under increased pressure. As is customary, thepolymerization can also be carried out in an inert gas atmosphere,preferably under nitrogen.

The quality properties of the polymers can be improved further bysubsequently heating the aqueous polymer gels in the temperature rangefrom 50° to 130° C., preferably 70° to 100° C., for several hours,preferably 8 to 15 hours.

The hydrophilic polymers according to the invention prepared by thisroute, which are in the form of aqueous gels, can be obtained andemployed in solid form by known drying processes, after mechanicalcomminution with suitable apparatuses.

The hydrophilic polymers according to the invention have considerablyhigher molecular weights than the known polymers of the prior art andhave significant advantages compared with these. In particular, theyhave a high liquid-bonding capacity coupled with simultaneously highliquid retention values and a high mechanical strength of the swollengel particles, with low extractable contents.

They are therefore outstandingly suitable as absorbents for water andaqueous liquids, such as urine or blood, in hygiene articles such asnappies for babies and adults, bandages, tampons and the like. However,they can also be used as soil-improving agents in agriculture andhorticulture, as moisture-binding agents in cable sheathing and forthickening aqueous waste products.

EXAMPLE 1

1.0 g (0.003571 mol) of 4,4'-azobis(4-cyanovaletic acid) was dissolvedin 100 g of dimineralized water at 50° C., 0.337 g (0.0019379 mol) ofethylene glycol diglycidyl ether was added and the mixture was left tostand at room temperature for 24 hours. 400 g of dimineralized waterwere initially introduced into a 1 l glass polymerization flask, 70 g(0.83 mol) of Na bicarbonate were suspended therein, and 200 g (2.77mol) of acrylic acid were added dropwise such that foaming over wasavoided. The monomer solution thereby cooled to about 10° C. Thepreviously prepared initiator solution was then transferredquantitatively to the reaction flask with the aid of 50 ml ofdimineralized water as a rinsing agent, and the components were stirredhomogeneously. The clear monomer solution was then left to stand under aCO₂ atmosphere at room temperature for 14 hours, without stirring andwithout a reaction being detectable. It was then diluted with 150 ml ofdimineralized water, rendered inert by passing in N₂ and heated to aninternal temperature of 50° C. When this temperature was reached, thepolymerization reaction started immediately and a high-viscosity pastewas formed, which was after-heated at 50° C. for 12 hours. A 0.1%strength solution of the polymer in demineralized water (based onacrylic acid) had a relative vis-cosity of 28.7335, measured in anUbbelohde capillary viscometer type Ic at 25° C.

For comparison, the process was repeated, but no ethylene glycoldiglycidyl ether was added to the initiator solution. A polymer of whicha 0.1% strength solution in demineralized water (based on acrylic acid)had a relative viscosity of 4.3345, measured in an Ubbelohde capillaryviscometer type Ic at 25° C., was obtained.

A significantly high molecular weight polymer, expressed by the relativeviscosity, was thus obtained by using the initiator which formsdi-radicals than by using the initiator which forms mono-radicals.

The resulting gel according to the invention and the comparison gel werekneaded with in each case 44.3% by weight of 50% strength NaOH (based onthe acrylic acid) in a kneader until homogeneous, in each case 0.5% byweight (based on the acrylic acid) of methylphosphonic acid diglycidylester was then added, the mixture was homogenized at temperatures of 70°to 80° C. and the composition, which was comminuted mechanically afterdischarge from the kneader, was dried in a stream of air at 180° C. Theproduct was ground and sieved (850/100 μm). The following performancedata were obtained:

    ______________________________________                                                  Extractables                                                                  1 h   16 h     CRC     FSC   AUL                                              [%]   [%]      [g/g]   [g/g] [g/g                                   ______________________________________                                        Gel according to                                                                          4.6     9.9      39    58    10.6                                 the invention                                                                 Comparison  7.3     16.2     34    54    8.2                                  ______________________________________                                         CRC = Centrifuge Retention Capacity                                           PSC = Free Swell Capacity                                                     AUL = Absorption Under Load [20 g/cm.sup.2                               

EXAMPLE 2

1 g (0.003571 mol) of 4,4'-azobis-4-cyanovaleric acid was dissolved in20 ml of dimethylformamide, 0.31 g (0.00178 mol) of ethylene glycoldiglycidyl ether was added and the mixture was heated to 50° C. and keptat 50° C. for 14 hours.

600 g of demineralized water were initially introduced into a 1 1polymerization flask, 200 g of acrylic acid were dissolved therein andthis solution was heated to an internal temperature of 50° C. whilepassing in N₂. The previously prepared initiator solution, warmed to 50°C. was then transferred quantitatively into the reaction flask. Afterhomogenization, the introduction of N₂ was stopped and the reactionsolution was left to stand, without stirring. The polymerizationreaction started immediately thereafter, a high-viscosity paste beingformed. The mixture was after-heated at 50° C. for 12 hours. A 0.1%strength solution of the polymer in demineralized water (based onacrylic acid) had a relative viscosity of 2.5902, measured in anUbbelohde capillary viscometer type Ic at 25° C.

For comparison, the process was repeated, but no ethylene glycoldiglycidyl ether was added to the initiator solution. A polymer whose0.1% strength solution in demineralized water (based on the acrylicacid) had a relative viscosity of 2.2894, measured in an Ubbelohdecapillary viscometer type Ic at 25° C., was obtained.

The product according to the invention and the comparison product weresubjected to post-crosslinking as described in Example 1. The followingperformance data were obtained:

    ______________________________________                                                  Extractables                                                                  1 h   16 h     CRC     FSC   AUL                                              [%]   [%]      [g/g]   [g/g] [g/g                                   ______________________________________                                        Product accord-                                                                           5.8     10.6     36    57    10.3                                 ing to the                                                                    invention                                                                     Comparison  8.1     16.9     33    54    8.3                                  ______________________________________                                    

EXAMPLE 3

Example 2 was repeated, but only 0.155 g (0.00089 mol) of ethyleneglycol diglycidyl ether was employed. A polymer of which a 0.1% strengthsolution in demineralized water (based on the acrylic acid) had arelative viscosity of 4.2890, measured in an Ubbelohde capillaryviscometer type Ic at 25° C., was obtained.

The product was subjected to post-crosslinking as described inExample 1. The following performance data were obtained:

    ______________________________________                                        Extractables                                                                  1 h       16 h    CRC          FSC  AUL                                       [%]       [%]     [g/g]        [g/g]                                                                              [g/g]                                     ______________________________________                                        5.7       10.3    35           58   9.6                                       ______________________________________                                    

EXAMPLE 4

134.5 g of 50% strength NaOH (degree of neutralization=55 mol %) werestirred slowly into a mixture of 344 g of demineralized water, 300 g ofice from demineralized water and 220 g of acrylic acid in awell-insulated polymerization flask while stirring and under adiabaticconditions. 1 g (0.45% by weight) of methylenebisacrylamide was added,nitrogen was passed into the solution, while stirring, and the mixturewas brought to 5° C. 0.19 g of 2,5-dimethylhexane 2,5-dihydroperoxide(product of PEROXID-CHEMIE GmbH, Germany) and then 0.55 g of a 1%strength aqueous ascorbic acid solution were added, the mixture wasstirred homogeneously, the stirrer was removed and the mixture was leftto stand while passing in further N₂. The reaction started after only afew minutes, and in the course thereof the temperature rose to a maximumof about 55° C. and a sliceable gel was formed. This was left to standunder the same conditions for about 6 hours and then comminutedmechanically, dried in a thin layer in a stream of air at 180° C.,ground and, if appropriate, sieved. A product was obtained which gavethe performance data shown in Table 1.

For comparison, the abovementioned synthesis was repeated, with thedifference that instead of 2,5-dimethylhexane 2,5-dihydroperoxide,ammonium peroxodisulphate was employed. The performance data arelikewise shown in Table 1.

EXAMPLE 5

Example 4, including the comparison, was repeated, but instead of 0.45%by weight, 0.87% by weight of methylenebisacrylamide was employed. Theperformance data can be seen from Table 1.

EXAMPLE 6

Example 4, including the comparison, was repeated, but instead of 0.45%by weight, 1.36% by weight of methylenebisacrylamide was employed. Theperformance data are to be found in Table 1.

EXAMPLE 7

The following were employed in accordance with Example 4:

220 g of acrylic acid neutralized with NaHCO₃ to a degree ofneutralization of 55 mol %.

0.3% by weight, based on the acrylic acid, of trimethylolpropanetriacrylate as the crosslinking agent. 0.068% by weight, based on theacrylic acid, of the reaction product of 2,2'-azobisisobutyronitrilewith butane-1,4-diol (Pinner synthesis analogously to Example 1b fromMakromol. Chem. 178, 2533 (1977)) as the free radical initiator.

For comparison, the synthesis was repeated, but instead of theabovementioned free radical initiator, only the starting substances forits preparation were added. The performance data are to be found inTable 1.

EXAMPLE 8

a) Preparation of the free radical initiator:

288 g (1 mol) of 2,2'-azobis-2-methyl-N-(2-hydroxyethyl)-propionamidewere introduced slowly into a solution of 600 ml of anhydrouscyclohexane and 140 g (1 mol) of hexamethylene diisocyanate and 0.3 g ofdibutyltin dilaurate, as the catalyst, while stirring, the reactiontemperature being kept constant at 0° to 10° C. by external cooling. Themixture was subsequently stirred at room temperature for 30 to 60minutes, and the precipitate formed was then filtered off and freed fromresidual solvent under reduced pressure at a maximum of 40° C.

b) Preparation of the polymer according to the invention:

The following were reacted in accordance with Example 4:

220 g of a mixture of acrylic acid and vinylphosphonic acid in a molarratio of 100:1, neutralized with NarCO₃ to a degree of neutralization of55 mol %.

0.3% by weight, based on the monomers, of trimethylolpropanetriacrylateas the crosslinking agent.

0.068% by weight, based on the monomers, of the initiator prepared inaccordance with a).

For comparison, the synthesis was repeated, but instead of theabovementioned free radical initiator, only the starting substances forits preparation were added. The performance data are to be found inTable 1.

EXAMPLE 9 a) Preparation of the Free Radical Initiator

A solution (slightly cloudy) of 23.6 g (0.1 mol) of2,2'-azobis(2-methylpropionamide) dihydrate in 500 g of water wasbrought to a weakly basic pH of 7.5 with dilute sodium carbonatesolution. 14.5 g (0.1 mol) of 40% strength aqueous glyoxal were thenadded and the reaction solution was heated to 40° C. and stirred at thistemperature for 6 hours. After being cooled to <20° C., the reactionsolution was employed directly for the polymerization experiments.

b) Preparation of the Polymer According to the Invention

The following were reacted in accordance with Example 4:

220 g of acrylic acid which, after the polymerization, was neutralizedwith NaOH to a degree of neutralization of 70 mol %.

0.45% by weight, based on the acrylic acid, of tetraallyloxyethane asthe crosslinking agent.

0.091% by weight, based on the acrylic acid, of the initiator preparedin accordance with a).

For comparison, the synthesis was repeated, but instead of theabovementioned free radical initiator, only the starting substances forits preparation were added. The performance data are to be found inTable 1.

EXAMPLE 10

The following were reacted in accordance with Example 4:

632 g of 2-acrylamido-2-methylpropanesulphonic acid, neutralized withNaHCO₃ to a degree of neutralization of 70 mol %.

0.1% by weight, based on the monomer, of tetraallyloxyethane as thecrosslinking agent.

0.046% by weight, based on the monomer, of the initiator preparedaccording to Example 9a).

For comparison, the synthesis was repeated, but instead of theabovementioned initiator, only the starting substances for itspreparation were added. The performance data are to be found in Table 1.

EXAMPLE 11

The following were reacted in accordance with Example 4:

220 g of acrylic acid, neutralized with NaHCO₃ to a degree ofneutralization of 55 mol %.

0.3% by weight, based on the acrylic acid, of trimethylolpropanetriacrylate.

0.022% by weight, based on the acrylic acid, of 2,5-dimethylhexane2,5-dihydroperoxide and 0.025% by weight, based on the acrylic acid, ofthe free radical initiator employed in Example 7.

For comparison, the synthesis was repeated, but instead of theabovementioned initiator mixture, only a mixture of2,2'-azobisisobutyronitrile and butane-1,4-diol was added. Theperformance data are to be found in Table 1.

EXAMPLE 12 a) Preparation of the Free Radical Initiator

Apparatus: double-walled glass beaker electrolysis cell having a groundglass adaptor on the side, Teflon stopper with bores for electrodes, gasinlet tube and thermometer; Pt sheet electrodes on holder; cryostat;galvanostat with current leads, measuring equipment and the like.

0.35 g of NaOH (0.0086 mol) was added to 150 g of an aqueous solutionwhich contained 8.3% (12.45 g, 0.173 molar equivalent of COOH) ofpolyacrylic acid (M_(w) =about 200,000), and the mixture was transferredto the electrolysis cell and temperature-controlled at 10° C. with theaid of a cryostat. A constant stream of O₂ was now passed into thesolution via the gas inlet tube, onto the lower end of which was fused aglass frit. Electrolysis was carried out under a current of 150 mA,while stirring, up to a charge throughput of 1800 C, the internaltemperature being kept at 10° C. and the electrolyte being flushedconstantly with oxygen. The electrolysate was employed in this formdirectly for the polymerization experiments.

b) Preparation of the Polymer According to the Invention

The following were reacted in accordance with Example 4:

220 g of acrylic acid which, after the polymerization, was neutralizedwith NaOH to a degree of neutralization of 68 mol %.

5.8% by weight, based on the acrylic acid, of the initiator preparedaccording to a).

The performance data are to be found in Table 1.

For comparison, the synthesis was repeated, the solution mentioned undera) being employed as the initiator without electrolysis. Nopolymerization took place under these conditions (Comparison 1).

The comparison was repeated, but 0.09% by weight, based on the acrylicacid, of ammonium peroxodisulphate additionally being added. Awater-soluble, non-swellable polymer was formed under these conditions(Comparison 2).

EXAMPLE 13

The synthesis according to Example 12 was repeated, 0.1% by weight,based on the acrylic acid, of methylenebisacrylamide additionally beingadded.

For comparison, the synthesis was repeated, but the solution mentionedunder a), without electrolysis, and 0.09% by weight, based on theacrylic acid, of ammonium peroxodisulphate were employed as theinitiator. The performance data are to be found in Table 1.

                  TABLE 1                                                         ______________________________________                                        Performance data of the polymers according to Examples 4 to 13.                         Extractables                                                                    1 h     16 h     CRC   PSC   AUL                                  Example     [%]     [%]      [g/g] [g/g] [g/g]                                ______________________________________                                        4               4.3     7.4    38    54    9.4                                4   Comparison  8.1     19.2   35    52    9                                  5               1.9     4.5    28    47    21.4                               5   Comparison  6.6     9.1    29    45    15.4                               6               1       2.9    22    40    25.3                               6   Comparison  4.3     7.5    21    39    20.9                               7               5.1     8.2    43    63    9                                  7   Comparison  9.9     20     39    60    8.1                                8               8.1     12.4   35    70    8.6                                8   Comparison  12      19.3   29    63    8.4                                9               6.3     8.1    31    55    11.4                               9   Comparison  7.5     9.7    26    49    9.9                                10              9.9     13.1   31    54    8.4                                10  Comparison  11.2    16.2   27    50    8.1                                11              4.5     8.2    44    58    10                                 11  Comparison  6.2     9.1    39    51    8.8                                12              7.6     16.3   26    58    8.3                                12  Comparison 1                                                                              no polymerization                                             12  Comparison 2                                                                              water-soluble polymer                                         13              5.6     14.4   28    57    9.7                                13  Comparison  7.3     16.9   25    56    8.1                                ______________________________________                                    

We claim:
 1. Water-swellable hydrophilic polymers which comprisepolymers prepared by free radical (co)polymerization of one or morehydrophilic monomers of the formula I ##STR5## wherein R¹ is hydrogen,methyl or ethyl,R² is the group --COOR⁴, the eulphonyl group, thephosphonyl group, the phosphonyl group esterified by (C₁ -C₄)-alkanol ora group of the formula ##STR6## R³ is hydrogen, methyl, ethyl or thecarboxyl group, R⁴ is hydrogen, amine or hydroxy-(C₁ -C₄)-alkyl and R⁵is the sulphonyl group, the phosphonyl group or the carboxyl group, orgrafting (co)polymerization of one or more hydrophilic monomers of theformula I onto a grafting base, using a free radical initiator whichforms three or more free radical sites per molecule, and whereincrosslinking is accomplished in the absence of a crosslinking agent. 2.Water-swellable hydrophilic polymers which comprise polymers prepared byfree radical (co)polymerization of one or more hydrophilic monomers ofthe formula I ##STR7## wherein R^(x) is hydrogen, methyl or ethyl,R² isthe group --COOR⁴, the sulphonyl group, the phosphonyl group, thephosphonyl group esterified by (C₁ -C₄)-alkanol or a group of theformula ##STR8## R³ is hydrogen, methyl, ethyl or the carboxyl group, R⁴is hydrogen, amino or hydroxy-(C₁ -C₄)-alkyl and R⁵ is the sulphonylgroup, the phosphonyl group or the carboxyl group, using a free radicalinitiator obtained by anodic oxidation of a polycarboxylic acid andwherein crosslinking is performed in the absence of a crosslinkingagent.
 3. Water-swellable hydrophilic polymers according to claim 1,wherein said free radical initiator is obtained by anodic oxidation ofpolycarboxylic acids in the presence of oxygen.
 4. The water-swellablehydrophilic polymers according to claim 2, further comprising acrosslinking agent and the resultant polymer would be water-swellableeven if the crosslinking agent had not been added.
 5. Thewater-swellable hydrophilic polymers according to claim 1, wherein saidhydrophilic monomers are selected from the group consisting of acrylicacid, methacrylic acid, vinylsulphonic acid, vinylphosphonic acid,maleic acid, fumaric acid, itaconic acid,2-acrylamido-2-methylpropanesulphonic acid and their amides,hydroxyalkyl esters, esters containing amino groups, amides containingamino groups and ammonium groups.
 6. The water-swellable hydrophilicpolymers according to claim 1, wherein said hydrophilic monomers areacrylic acid or methacrylic acid.
 7. The water-swellable hydrophilicpolymers according to claim 1, further comprising a crosslinking agentand the resultant polymer would be water-swellable even if thecrosslinking agent had not been added.
 8. The water-swellablehydrophilic polymers according to claim 1, wherein said grafting base isstarch, cellulose, polysaccharide, oligosaccharide, polyalkaline oxideor hydrophilic polyesters.
 9. Water-swellable hydrophilic polymersaccording to claim 1, wherein said grafting base is starch,polyethylene-oxide or polypropylene-oxide.
 10. Water-swellablehydrophilic polymers according to claim 1, wherein said free radicalinitiator is a compound which contains at least three hydroperoxideunits, peroxide units or azo units.
 11. Water-swellable hydrophilicpolymers according to claim 1, wherein said free radical initiator is areaction product of 4,4'-azobis (4-cyanovaleric acid) with apolyglycerol polyglycidyl ether.
 12. The water-swellable hydrophilicpolymers according to claim 1, wherein said free radical initiator's arepresent in amount from about 0.001 to about 20% by weight based on thetotal monomers.
 13. The water-swellable hydrophilic polymers accordingto claim 1, wherein said free radical initiator's are present in amountfrom about 0.05 to about 3% by weight based on the total monomers. 14.Water-swellable hydrophilic polymers according to claim 1, furthercomprising a crosslinking agent.
 15. The water-swellable hydrophilicpolymers according to claim 14, wherein said crosslinking agent isselected from the group consisting of methylenebisacrylamide,methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids with polyols, allyl (meth)acrylate, triallylcyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane,triallylamine, tetraallylsthylenediamine, allyl esters of phosphoricacid and vinylphosphonic acid derivatives.
 16. The water-swellablehydrophilic polymers according to claim 15, wherein said crosslinkingagent is present is an effective amount to provide crosslinkingproperties up to about 20% by weight based on the total monomer content.17. The water-swellable hydrophilic polymers according to claim 15,wherein said crosslinking agent is present is an effective amount toprovide crosslinking properties up to about 3% by weight based on thetotal monomer content.
 18. An absorbent comprising the water-swellablehydrophilic polymer according to claim
 1. 19. Hygiene articlescomprising said absorbent as claimed in claim 18, mixed with water oraqueous liquids.